In a pre-recorded interview, which aired yesterday in the US Mr Obama also called the Secret Service agents embroiled in the Colombian sex scandal "knuckleheads".

Allegations have been made that US Secret Service agents have been involved in hiring prostitutes ahead of an international summit in Colombia.

At least 12 Secret Service members and 12 US military personnel have been investigated in connection with an incident on the night of April 11-12 when the men are alleged to have taken as many as 21 prostitutes back to their hotel in Cartagena ahead of a presidential visit.

Mr Obama said he did not know what the men were thinking, but insisted that the vast majority of perform their work admirably

He also joked that he and presumptive Republican presidential candidate Mitt Romney had met "but we're not friends".